Best concert you've attended

Creedence and Booker T and the Mrs Madison Square Garden 1970

June, 1990 - Lakewood Amphitheater, Atlanta - Outdoor venue.

Stevie Ray Vaughn

BB King

Joe Cocker

I was 24. It was a perfect early summer night. Each band performed their own respective sets over a couple hours. At the end, everyone joined for a jam on stage. Imagine Stevie Ray Vaughn trading guitar licks with BB King, while a wildly contorted Joe Cocker inserted powerful, gravel-toned vocals throughout! It was fantastic!

Sadly, SRV was killed in a helicopter crash just 2 months later.

Come on, you have to at least let us have two. 

Harry Chapin,  January 20, 1981 at the MSU Auditorium, East Lansing, MI.  Amazing performer, he broke 3 guitar strings during the show. 

Jane Siberry, December 11, 1999, at Cafe Largo, Los Angeles, I'll never forget her alternate version of the 12 days of Christmas that was hilarious and beautiful.  

The Tragically Hip, 1993?.. headliners on a festival weekend. Summer time, a large open grassy field just outside Vancouver BC with snow covers mountain peaks on each side. Magical setting. The band had everyone in the palm of their hands. AWAESOME... Oh ya, and Midnight Oil opened for them..

I was going to list all the performers I've seen that were already mentioned by other respondents here but that list got too long ... I'll just go with the original premise:

Best concert I've attended was my favorite group's farewell concert - The Last Waltz